ENH: reduce reliance on FOAM_EXT_LIBBIN during builds

- just check WM_PROJECT_DIR instead.

- provide a fallback value when FOAM_EXT_LIBBIN might actually be needed.

Only strictly need FOAM_EXT_LIBBIN for scotch/metis decomposition, and
when these are actually supplied by ThirdParty.
All other ThirdParty dependencies are referenced by BOOST_ARCH_PATH etc.

Can therefore drop the FOAM_EXT_LIBBIN dependency for VTK-related
things, which do not use scotch/metis anyhow.